1. Field of the Invention
The present invention relates to a wireless communication system, and in particular, to a multi-input multi-output (MIMO) system for wireless communication.
2. Description of the Prior Art
Recently, due to the rapid increase in the requirements of wireless communication, academic circles and industrial manufacturers have been continuously researching transmission methods for highly efficient communication. According to basic communication theory, the simplest method for achieving highly efficient communication is to increase the bandwidth of signal transmission. As the bandwidth is restricted and limited, however, it is futile to claim greater efficiency by using a very wide bandwidth to transmit a lot of data. Consequently, a most interesting research subject is how to use a plurality of antennas for transmitting and receiving data under limited bandwidth to achieve highly efficient data communication. That is, in a particular circumstance with limited bandwidth, the amount of transmitted data can be raised through increasing the number of transmitting and receiving antennas.
Because different data is assigned to different antennas for transfer at the same time and at the same bandwidth, these signals transferred by different antennas will obviously interfere with each other at the receiving end. Therefore, a receiver utilizes a plurality of antennas to receive signals delivered by a plurality of antennas of a transmitter. As every antenna of the receiver receives signals transferred by a different antenna of the transmitter, however, the receiver cannot identify the signal received by one antenna unless the receiver executes signal processing. Please refer to FIG. 1. Assume a transmitter 10 includes M antennas and a receiver 20 includes N antennas. The transmitter 10 delivers M symbols X1, X2, . . . , XM during one symbol duration, and then these symbols pass the channel and are received by N antennas of the receiver 20. y1, y2, . . . yN represents signals received by different antennas at the same time, so the relationship between a transmission signal, a receiving signal, and the channel is described through vectors as the following:
                              Y          =                      HX            +            W                          ⁢                                  ⁢        wherein        ⁢                                  ⁢                              Y            =                          [                                                                                          y                      1                                                                                                                                  y                      2                                                                                                            ⋮                                                                                                              y                      N                                                                                  ]                                ,                      X            =                          [                                                                                          x                      1                                                                                                                                  x                      2                                                                                                            ⋮                                                                                                              x                      M                                                                                  ]                                ,                      W            =                          [                                                                                          w                      1                                                                                                                                  w                      2                                                                                                            ⋮                                                                                                              w                      N                                                                                  ]                                ,                                          ⁢                      H            =                          [                                                                                          h                      11                                                                                                  h                      12                                                                            ⋯                                                                              h                                              1                        ⁢                        M                                                                                                                                                        h                      21                                                                                                  h                      22                                                                            ⋯                                                                              h                                              2                        ⁢                        M                                                                                                                                  ⋮                                                        ⋮                                                        ⋱                                                        ⋮                                                                                                              h                                              N                        ⁢                                                                                                  ⁢                        1                                                                                                                        h                                              N                        ⁢                                                                                                  ⁢                        2                                                                                                  ⋯                                                                              h                      NM                                                                                  ]                                                          eq        .                                  ⁢        1            
W represents noise received by N antennas of the receiver 20, and H a represents signals transmitted by different antennas of the transmitter 10 passed to several possible channels to be received by the receiver 20. In detail, hij represents the channel for signal transmission from antenna j of the transmitter 10 to antenna l of the receiver 20, and at the point of communication, symbols X1, X2, . . . , XM delivered by the transmitter 10 may be BPSK, QPSK, 4-QAM, 16-QAM, or other modulation types. Consequently, the purpose of the receiver 20 is to properly process the signal Y received by antennas of the receiver 20 in order to identify symbols X1, X2, . . . , XM delivered by M antennas of the transmitter 10. Furthermore, related art is disclosed in U.S. Pub. No. 2003/0076890, “method and apparatus for detection and decoding of signals received from a linear propagation channel” (this data is incorporated herein by reference). The related art has significant limitations, however.
The receiving end still has the unresolved issue of properly processing the receiving signal Y received by antennas of the receiver in order to obtain symbols delivered by M transmission antennas.